This subproject 4 will build upon findings from the previous funding period to: 1) conduct an extensive analysis of the intercommunication between nucleus accumbens (NAcc) and ventral pallidum (VP) during cocaine and speedball (cocaine/heroin combinations) self-administration; 2) incorporate novel models of drug self-administration to characterize the transition from drug use to drug abuse which may more closely mimic the addictive processes in humans. The investigators have used 6-hydroxy-dopamine (6-OHDA) and D2 receptor alkylation to reveal differences in cocaine and speedball self-administration in NAcc and VP. These data, and recent literature, suggest important differences between NAcc and VP in the maintenance of drug self-administration. The first specific aim will characterize the role of D2 receptors in the NAcc and VP in the neurochemical effects of cocaine and speedball using in vivo microdialysis. The second specific aim will characterize the relative efficacy of cocaine, heroin and speedball with a discrete trial model of self-administration that shows significant promise for the investigation of the neurobiological events that underlie the transition from drug use to the loss of control that characterizes drug abuse. Cocaine, heroin and speedball will be evaluated for their abilities to maintain self-administration in a circadian or non-circadian manner as the discrete trial interval is decreased. Food reinforced responding will be evaluated concurrently as another measure of the loss of control. Microdialysis will be used to determine if findings from Specific Aim 1 are altered in this model of excessive drug use. The third specific aim will characterize the role of D2 receptors in NAcc and VP in the discrete trial self-administration of cocaine, heroin or speedball. D2 receptors in either NAcc, VP or both regions will be depleted in rats self-administering cocaine, heroin or speedball with discrete trial access. The effects on pattern of self-administration will be assessed to determine if D2 receptor depletion will restore circadian patterns of drug intake and/or reverse the effects on disruption of food-maintained responding. These brain regions will then be assessed for gene expression using real-time RT-PCR for relevant identified targets. Brain tissue will be shared with subprojects 0001, 0008 and 0011 for receptor G-protein coupling studies, connexin gene expression and for ex vivo voltammetry. In addition, brain tissue will be received from Project 0011 for targeted gene expression studies using real-time RT-PCR when relevant targets are identified using the tissue rom animals in specific aims 2 and 3. The proposed experiments will hopefully add significantly to our understanding of NAcc - VP interactions in drug self-administration and in the transition from drug use to drug addiction.